Variables in C#

Reading

Learning Objectives

Introduce students to writing and running their first programs.
Introduce students to the basic types of variables and how to use them.
Introduce students to reading from the console and writing information to the console.

What is a variable?

Computers are designed to accept, manipulate, and return information. In order for our code to accept, manipulate and return information we need to keep track of it. We store this information in our computer's memory and need to keep track of where we assigned each piece of information. This is where a variable comes into play.

A variable is:

an identifier for a value our program needs to keep track of

We can imagine the computer's memory as a large area to store information and the variables as labels, or pointers, to that spot in memory.


                                 +------------------------------------------------------------------+
                                 |                           COMPUTER MEMORY                        |
                                 |                                                                  |
                                 |                                                                  |
                                 |                                                                  |
                                 |               +---------------------------------------+          |
                                 |               |                                       |          |
                                 |               |                                       |          |
                                 |               |    "Suncoast Developers Guild"        |          |
myVariable  +------------------------------------+                                       |          |
                                 |               |                                       |          |
                                 |               +---------------------------------------+          |
                                 |                                                                  |
                                 |                                                                  |
                                 |          +-------+                                               |
                                 |          |       |                                               |
 myOtherVariable  +-----------------------> |  42   |                                               |
                                 |          |       |                                               |
                                 |          +-------+                                               |
                                 |                                                                  |
                                 |                                                                  |
                                 |                                                                  |
                                 |                                                                  |
                                 |                                                                  |
                                 +------------------------------------------------------------------+

So instead of repeating a value over and over in our code we can assign it to a variable.

Each language you learn will have specific rules about:

Naming variables
The kinds (types) of information we can store in variables
If the value of a variable, once assigned, can be changed
The syntax for declaring a variable
If the kind (type) of information stored in a variable can change once assigned

In this lesson we will look at how the C# language works with variables and we will learn a few, though not all, types of information our variables can store.

Declaring Variables

In C# we must declare our variables to our code before we can use them. By declare we mean to write a specific line of code, or instruction, to tell the C# program that we are introducing a new variable.

However, in C# we must also tell the program the kind (type) of information we want to store.

Let's say we wanted to keep track of two pieces of information in our program. We want to keep track of the name of a student and the score they received on their test.

Before we declare our variables we need to think about the kind of information each variable will represent. In this case, the name of the student is what we call a string. In programming terms a string is a sequence of letters, digits, spaces, punctuation, as well as other kinds of symbols that as a whole we call characters. So for keeping track of the student's name a string would be the best kind of variable. For the student's grade we will allow all the numbers between 0 and 100. In programming these kinds of values, numbers without any decimal places, are referred to as integers or in C# we use the word int. These values can be negative, zero, or positive, however they are not infinite. The computer can only keep track of an int that is so big. The range of an int variable is from -2,147,483,648 to 2,147,483,647 and since 0 to 100 fits nicely, we'll use an int to keep track of the student's score.

Let's declare these two variables with the following statements.

NOTE: For this first example we are going to include the entire contents of Program.cs but from this point forward we'll only show the code that goes inside Main


using System;
namespace our_dotnet_app
{
    class Program
    {
        static void Main(string[] args)
        {
          string name = "Mark";
          int score = 95;
        }
    }
}

Let's break down each of these lines to see the structure of a line of code in C# and how variable assignment works.


// Type   Name   Value    Statement End
//  |       |      |       |
//  |       |      |       |
//  v       v      v       v
  string  name = "Samantha";

This is an assignment statement. On the left hand side of the statement we have the type of variable we are asking to create, in this case a string followed by the name of the variable, in this case name, followed by an equals sign = and then the value, the string "MARK". Strings are enclosed inside a pair of double-quotation marks (") to let the computer know that we mean the letters M, then a, then r, then k. Otherwise the computer might think we are referring to another variable, one named Mark. Like all statements in C# we end (terminate) the statement with a semi-colon, ;. Think of the ; in C# like the period at the end of this sentence. It signifies the end of a complete thought. In the case of our code, the complete statement declaring our variable.

Looking at the other statement int score = 95; we see the same pattern. In this case we are declaring an integer variable named score and we make its value the number 95. Notice we do not put the 95 in any kind of quotes or other syntax. We simply refer to it by its value.

Let C# do the work

You will notice that developers often like it when the computer can do some of the work. Perhaps you noticed from our declarations we repeated some of the ideas. We said that we had to write string to let the computer know we were making a string variable, but then we also said that we had to write Mark inside double-quotes to let the computer know this was a sequence (string) of letters. The same with the int for the score.


string name = "Mark";
int score = 95;

The C# language can, when declaring a variable, infer (determine on its own) the type of variable if we supply it with an initial value. Adding an initial value for a variable is a good idea anyway since without it, the computer will assign an undefined value, which we'll get into later.

So if providing an initial value will allow the computer to determine the type of variable for itself, there must be a way to let it do that? This is where the keyword var comes into play.

Let's look at the same statement without having to explicitly (doing it ourselves) declare variables:


var name = "Mark";
var score = 95;

These two statements end up being the same as the previous example. However, the second example is easier for us to write. This is the style we will use most of the time when declaring new variables.

Advanced note: This is called type inference in a programming language.

What are the simple types of variables we can declare?

In C# there is a set of built-in types of variables that the language provides. We call these the primitive types.

The common numeric primitives are:
- int : a whole number from -2,147,483,648 to 2,147,483,647
- long : a whole number from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
- double : a 64 bit floating point value that has an approximate precision of ~15-17 digits
- float : a 32 bit floating point value that has an approximate precision of 6-9 digits
- decimal : a more precise way to store decimal numbers, but has a smaller range
Some other non-numeric primitives are:
- char : represents a unicode character
- bool : true or false
- byte : represents a raw chunk of data (values from 0 to 255)

NOTE: Do not feel the need to memorize all the different primitive types, just know that they exist. Most of the time developers will just var and not think about the type until it matters. This is a case of knowing how to find the information is better than rote memorization of facts.

Variable Examples

Let's see some additional examples of declaring the types of variables we have seen so far:

Integers


var score = 42;

Floats


var total = 10f;
var price = 10.0;
var pi = 3.14159265;

Decimal


var total = 10m;
var price = 10.0;
var pi = 3.14159265;

Double


var total = 10.0;
var total = 10d;
var pi = 3.14159265

Characters


var piratesFavoriteLetter = 'r';
var firstLetterOfTheEnglishAlphabet = 'a';
var lastLetterOfTheEnglishAlphabet = 'z';
var capitalLetter = 'Q';

Strings


var name = "Mark";

Strings have extra features!

You may have noticed that our string type wasn't amongst the list of primitive types. This is because it is a more advanced type since it has specific features.

How long is a string?

With a more complex type like a string we can ask it questions about itself. For instance, we can ask the variable how many characters are in the string.


var sentence = "The quick brown fox jumped over the lazy dog";
var howLong = sentence.Length;

With the code var howLong = sentence.Length; we see a variable being used on the right hand side for the first time. When processing an assignment statement the computer will figure out the value of the right hand side first before taking that result and assigning it to the variable on the left. The next new syntax here is sentence.Length -- the . in that part of the statement separates the variable (sentence) from the property we are asking about, Length. So to break this down:


    variable  right-hand-side
       |             |
       |             |
       v             v
var howLong = sentence.Length;


variable  property
   |         |
   |         |
   v         v
sentence.Length

If we ask the sentence variable for its Length, the computer first goes to the contents of the variable in memory, in this case the string The quick brown fox jumped over the lazy dog and then looks at what property we want, Length. In this case we get the _int_eger 44

So after:


var sentence = "The quick brown fox jumped over the lazy dog";
var howLong = sentence.Length;

The howLong variable will know it is an integer, and it's contents will be 44

Before we go we should look at a few more things that strings can do. Since we mentioned that strings are just a sequence of characters there should be a way to access each letter individually.

By using the [] syntax for a string we can access individual characters of the string.


var sentence = "The quick brown fox jumped over the lazy dog";
var firstLetter = sentence[0];
var secondLetter = sentence[1];
var thirdLetter = sentence[2];

So the number inside the [] brackets indicates which position of the string we want the character for. We call this the index of the character. You may also notice that the first character of the string is at index 0. Most languages begin counting at 0 and you'll see this fairly often. It will help you if you think of this as the index as opposed to the 1st, 2nd, etc. So the character at index 0 instead of the 1st character

DateTime

C# comes with a specialized class to deal with storing a date and time. A DateTime is a Year, Month, Day, Hour, Minute, Second, and Millisecond.

To create a DateTime variable equal to the current time you can use this syntax:


var rightNow = DateTime.Now

If you then wanted to know the current month:


var rightNow = DateTime.Now;
var currentMonth = rightNow.Month;

You can see a full list of the properties of a DateTime on Microsoft's Documentation for DateTime

Conclusion

Variables are a fundamental aspect of many programming languages. C# provides many built in data types as well as type inference so we can use the var keyword to make variable creation easier.

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